Method of Forming a Flash NAND Memory Cell Array With Charge Storage Elements Positioned in Trenches

ABSTRACT

NAND arrays of memory cells are described, as well as methods of forming and using them. Memory cell charge storage devices, such as conductive floating gates, are oriented vertically in trenches, with control gates positioned both in the trenches between charge storage elements and over a horizontal surface between the trenches. Individual charge storage devices are therefore field coupled with two control gates, one on either side.

CROSS-REFERENCE TO RELATED PATENTS AND APPLICATIONS

This application is related to concurrently filed application entitled“Flash NAND Memory Cell Array With Charge Storage Elements Positioned inTrenches” by Nima Mokhlesi, which is incorporated herein in its entiretyby this reference.

This application is related to U.S. applications Ser. Nos. 11/533,313and 11/533,317 of Nima Mokhlesi, both filed Sep. 19, 2006.

This application is also related to U.S. Pat. Nos. 6,888,755 and7,075,823 of Eliyahou Harari.

BACKGROUND

This invention relates generally to non-volatile flash memory systems,and, more specifically, to a memory cell structure and process offorming arrays of memory cells that utilize trenches to reduce theoverall size of the arrays.

There are many commercially successful non-volatile memory productsbeing used today, particularly in the form of small form factor cards,which use an array of flash EEPROM (Electrically Erasable andProgrammable Read Only Memory) cells. Arrays with either a NOR or a NANDarchitecture are commonly used. One or more integrated circuit chipscontaining a memory cell array are commonly combined with a controllerchip to form a complete memory system. Alternatively, part or all of thecontroller function may be implemented on the same chip that containsall or part of the memory cell array.

A memory cell array with the NAND architecture utilizes series stringsof more than two memory cells, such as 16 or 32, connected along withone or more select transistors between individual bit lines and areference potential to form columns of cells. Word lines extend over thememory cells in a row direction, across a large number of these columns.An individual cell within a column is read and verified duringprogramming by causing the remaining cells in the string to be turned onhard so that the current flowing through a string is dependent upon thelevel of charge stored in the addressed cell. Examples of NANDarchitecture arrays and their operation as part of a memory system arefound in U.S. Pat. Nos. 5,570,315, 5,774,397, 6,046,935, 6,522,580,6,888,755 and 6,925,007.

There are various programming techniques for causing electrons to travelthrough the gate dielectric from the semiconductor base structure andonto the charge storage element. The most common programming mechanismsare described in a book edited by Brown and Brewer, “NonvolatileSemiconductor Memory Technology,” IEEE Press, section 1.2, pages 9-25(1998), where the charge storage elements are conductive floating gates.One technique, termed “Fowler-Nordheim tunneling” (section 1.2.1),causes electrons to tunnel through the floating gate dielectric underthe influence of a high field that is established thereacross by avoltage difference between the control gate and the semiconductorstructure channel. Another technique, channel hot electron injection inthe drain region, commonly referred to as “hot-electron injection”(section 1.2.3), injects electrons from the cell's channel into a regionof the floating gate adjacent the cell's drain. Yet another technique,termed “source side injection” (section 1.2.4), controls thesemiconductor structure surface electrical potential along the length ofthe memory cell channel in a manner to create conditions for electroninjection in a region of the channel away from the drain. Source sideinjection is also described in an article by Kamiya et al., “EPROM Cellwith High Gate Injection Efficiency,” IEDM Technical Digest, 1982, pages741-744, and in U.S. Pat. Nos. 4,622,656 and 5,313,421. In a furtherprogramming technique, termed “ballistic injection” high fields aregenerated within a short channel to accelerate electrons directly ontothe charge storage element, as described by Ogura et al., “Low Voltage,Low Current, High Speed Program Step Split Gate Cell with BallisticDirect Injection for EEPROM/Flash”, IEDM 1998, pages 987-990.”

Charge is removed from charge storage elements to erase the memorycells. According to one technique, the memory cells are erased to thebase semiconductor structure by applying appropriate voltages to thesource, drain, semiconductor structure and other gate(s) to causeelectrons to tunnel through a portion of a dielectric layer between thecharge storage elements and the base structure. A large number of memorycells are grouped together into blocks of a minimum number of cells thatare simultaneously erased, in a “flash.” Individual blocks can store anumber of pages of data, each page containing one more host sectors'worth of data. The pages are individually programmable and readable.Examples of operating large block memories are given in U.S. Pat. No.6,968,421.

Currently, electrically conductive floating gates are the most popularform of charge storage elements used in the memory cells. But some flashmemories utilize a non-conductive dielectric material that trapselectrons. In either case, an individual memory cell includes one ormore charge storage elements. Examples of the use of dielectric aredescribed in U.S. Pat. No. 6,925,007 and documents referenced therein.In the case of a dielectric trapped charge memory cell, two or morecharge storage elements may be formed as two or more regions of a singlecontinuous layer of dielectric that are spaced apart thereacross. Oneexample of a suitable charge storage dielectric material is athree-layer oxide-nitride-oxide (ONO) composite. Another example is asingle layer of silicon rich silicon dioxide material.

As in almost all integrated circuit applications, the pressure to shrinkthe area required to implement some integrated circuit function alsoexists with flash EEPROM systems. It is continually desired to increasethe amount of digital data that can be stored in a given area of asilicon structure, in order to increase the storage capacity of a givensize memory card and other types of packages, or to both increasecapacity and decrease size. Another way to increase the storage densityof data is to store more than one bit of data per memory cell. This isaccomplished by dividing a window of a memory cell threshold voltagerange into more than two ranges. The use of four such ranges allows eachcharge storage element to store two bits of data, eight ranges storesthree bits of data per charge storage element, and so on. A multiplestate flash EEPROM structure and operation is described in U.S. Pat.Nos. 5,043,940 and 5,172,338, as examples.

Some binary memory systems designate the erased state as a data logical“0”, with a programmed state being designated to be a logical “1”. Otherbinary systems follow an opposite convention, the erased state beingdesignated to be a “1” and the programmed state to be a “0”. In amulti-state system, one having four states per storage element forexample, the erased state may be designated as “00” and a maximallyprogrammed state as “11”. Others may reverse these designations.

SUMMARY

An array of memory cells is formed with the use of trenches or cavitiesin which the charge storage elements are positioned along sidewallsthereof and extended upwards a distance out of the trenches. Controlgates are positioned between adjacent charge storage elements in onedirection across the array, alternately in the trenches and in spacesbetween the trenches. In one example, the control gates are connected toconductors extending in an orthogonal direction across the array. In aNAND array, a plurality of such memory cells are connected in series inthe one direction, the series string being connectable between one ofmultiple bit lines and a common potential, and the control gateconductors that extend in an orthogonal direction function as wordlines.

The charge storage elements are most commonly conductive floating gatesthat remain within the area of the trenches in plan view. Spacers may beused as the floating gates, wherein a layer of floating gate material,such as conductively doped polysilicon, is isotropically deposited overthe array and into the trenches, followed by anisotropically etchingaway all of the deposited floating gate material except for spacers thatremain along the trench sidewalls. Conductive control gates can then beformed in the trenches between the floating gate spacers and in spacesbetween the trenches, so that the floating gates are individually fieldcoupled with a pair of control gates on opposite sides of the floatinggate. The extension of the floating gates out of the trenches allowssuch coupling with the control gates positioned between the trenches tobe made and controlled.

An overall advantage of this structure is its efficient use of thesurface area of a wafer or other base semiconductor structure. Asignificant portion of the length of the memory cell channel isvertically oriented along the trench walls, thus reducing the amount ofsurface area required for each memory cell.

Various aspects, advantages, embodiments and features of the presentinvention are included in the following description of exemplaryexamples thereof, whose description should be taken in conjunction withthe accompanying drawings.

All patents, patent applications, articles, specifications, standards,book portions, other publications, documents and things referencedherein are hereby incorporated herein by this reference in theirentirety for all purposes. To the extent of any inconsistency orconflict in the definition or use of terms between any of theincorporated publications, documents or things and the presentapplication text, those of the present application text shall prevail.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates in block diagram form a flash memory system;

FIG. 2 is a plan view of a portion of a NAND memory cell array of thememory system of FIG. 1;

FIG. 3 is a sectional view of the NAND memory cell array of FIG. 2according to a general first embodiment, taken at section A-Athereacross;

FIG. 4 is an equivalent circuit diagram of one string of memory cells inthe array of FIG. 2;

FIGS. 5-13 illustrate successive steps in a process of making the NANDmemory cell array of FIG. 2 according to a more specific secondembodiment, views A being taken at section A-A thereacross, views Bbeing taken at section B-B thereacross and views C being taken atsection C-C thereacross;

FIG. 14 is a final view of the process of making the second embodimentNAND array, taken at section A-A of FIG. 2; and

FIG. 15 is a top view of a few memory cells of the second embodimentNAND array.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS Memory System

An example memory system is illustrated by the block diagram of FIG. 1.A memory cell array 1 including a plurality of memory cells M arrangedin a matrix is controlled by a column control circuit 2, a row controlcircuit 3, a c-source control circuit 4 and a c-p-well control circuit5. The memory cell array 1 is, in this example, of the NAND type that isdescribed above in the Background and in references incorporated hereinby reference. A control circuit 2 is connected to bit lines (BL) of thememory cell array 1 for reading data stored in the memory cells (M), fordetermining a state of the memory cells (M) during a program operation,and for controlling potential levels of the bit lines (BL) to promotethe programming or to inhibit the programming. The row control circuit 3is connected to word lines (WL) to which the following voltages may beapplied: read voltages, program voltages combined with the bit linepotential levels controlled by the column control circuit 2, and anerase voltage coupled with a voltage of a p-type region on which thememory cells (M) are formed. The c-source control circuit 4 controls acommon source line (labeled as “c-source” in FIG. 1) connected to thememory cells (M). The c-p-well control circuit 5 controls the c-p-wellvoltage.

The data stored in the memory cells (M) are read out by the columncontrol circuit 2 and are output to external I/O lines via an I/O lineand a data input/output buffer 6. Program data to be stored in thememory cells are input to the data input/output buffer 6 via theexternal I/O lines, and transferred to the column control circuit 2. Theexternal I/O lines are connected to a controller 9. The controller 9includes various types of registers and other memory including avolatile random-access-memory (RAM) 10.

Command data for controlling the flash memory device are inputted tocommand circuits 7 connected to external control lines that areconnected with the controller 9. The command data informs the flashmemory of what operation is requested. The input command is transferredto a state machine 8 that controls the column control circuit 2, the rowcontrol circuit 3, the c-source control circuit 4, the c-p-well controlcircuit 5 and the data input/output buffer 6. The state machine 8 mayalso output status data of the flash memory.

The controller 9 is connected or connectable with a host system such asa personal computer, a digital camera, or a personal digital assistant.It is the host that initiates commands, such as to store or read data toor from the memory array 1, and provides or receives such data,respectively. The controller converts such commands into command signalsthat can be interpreted and executed by the command circuits 7. Thecontroller also typically contains buffer memory for the user data beingwritten to or read from the memory array. A typical memory systemincludes one integrated circuit chip 11 that includes the controller 9,and one or more integrated circuit chips 12 that each contain a memoryarray and associated control, input/output and state machine circuits.Alternatively, the memory array and controller circuits of a system maybe integrated together on one or more integrated circuit chips.

The memory system of FIG. 1 may be embedded as part of the host system,or may be included in a memory card or flash drive that is removablyinsertable into a mating receptacle of a host system. Such a card mayinclude the entire memory system, or the controller and memory array,with associated peripheral circuits, may be provided in separate cards.Several card implementations are described, for example, in U.S. patentsand published applications nos. U.S. Pat. Nos. 5,887,145, 6,820,148,7,114,659, 2005/0230483A1 and 2005/0230484A1.

General NAND Array Embodiment

Major components of a few memory cells of a NAND array are illustratedin plan view in FIG. 2. Five strings 21-25 of series connected memorycells are included, with six floating gate charge storage elements shownin each partial string. The string 23, for example, includes floatinggates 27-32. A row of charge storage elements, for example, includesfloating gates 33, 34, 30, 35 and 26, one in each of the five strings21-25. Only a small rectangular array of memory cells is illustrated forease of explanation, thirty charge storage elements being included.Actual implementations of such an array include millions of such memorycells in thousands of NAND strings, each string normally having 16, 32or more memory cells.

Each of the NAND strings 21-25 of FIG. 2 includes two selecttransistors, one at each end of the string, to controllably connect thestring between a different one of global bit lines BL0-BL4 and areference potential V_(S). V_(S) is normally ground during read but mayassume a small positive value during programming to assist in minimizingleakage across the source select transistor. A voltage V_(SSL) isapplied to a gate line 43 to cause select transistors at one end of theNAND strings to become conductive and connect the string ends to acommon potential V_(S). The other ends of the strings 21-25 areconnected through respective select transistors to the respective bitlines BL0-BL4 by voltage V_(DSL) applied to a common gate line 45. Thecolumn control circuits 2 (FIG. 1) apply a voltage to each bit line thatis representative of the specific data to be written, or sense thevoltage or current during a read operation.

In the NAND array shown in FIG. 2, control gate (word) lines 71-77extend across a multiple strings of memory cells between their floatinggates. The control lines are connected to the gates that areindividually coupled through a suitable insulating dielectric, such asmulti-layer oxide-nitride-oxide (ONO), to the floating gates on eitherside of the control gate lines. The control gate lines 71-77 areelongated in the x-direction across the array and separated in they-direction by the thickness of the intervening floating gates and thethicknesses of the dielectric layers between them.

This can be seen more clearly by reference to FIG. 3, which is across-sectional view through a few of the memory cells of the string 23of FIG. 2. Two trenches 81 and 83 are formed in a surface 87 of asemiconductor structure 85. The structure 85 can be a standard siliconwafer with a layer adjacent its surface 87 that is appropriately doped.Alternatively, the trenches may be formed in an epitaxial layer grown ona supporting substrate, or in a top semi-conductive layer of asilicon-on-insulator (SOI) wafer, as other examples of semiconductorbase structures. An SOI wafer has a dielectric layer between a top dopedlayer and an underlying mechanical supporting substrate. Floating gates28 and 29 are positioned along opposite sidewalls of the trench 81, withsuitable respective dielectric layers 91 and 93 therebetween. Similarly,floating gates 30 and 31 in the trench 83 have respective layers 95 and97 of dielectric between them and the structure 85. The trenches arepreferably formed with substantially vertical side walls in at least thedirection along the memory cell strings that is shown in FIG. 3.

A first set of control gates 101 and 103 is formed within respectivetrenches 81 and 83, separated from the floating gates on either sidethereof by dielectric layers 105, 107, 109 and 111. The control gate 101is field coupled with both of the floating gates 28 and 29. Similarly,the control gate 103 is field coupled with both of the floating gates 30and 31. The control gates 101 and 103 are electrically connected withrespective control gate lines 75 and 73. The control gates 101 and 103are separated from bottom surfaces of their respective trenches 81 and83 by dielectric layers 115 and 117.

A second set of control gates 119, 121 and 123 is formed betweentrenches along the direction of the memory cell string, over the surface87 with a layer 125 of dielectric therebetween. These control gates areindividually field coupled with the floating gates on either side of it.The control gate 121, for example, is coupled with the floating gates 29and 30 through dielectric layers 93 and 95, which may be supplemented byan additional dielectric thickness above the surface 87, as shown. Thecontrol gates 119, 121 and 123 are electrically connected to respectivecontrol lines 76, 74 and 72. The floating gates 28-31 extend out oftheir respective trenches 81 and 83 a distance above the surface 87, aconvenient way to provide surface area that is coupled with edges of thecontrol gates 119, 121 and 123 that are formed above the surface 87.

The control gates 101, 103 etc. of the first set are referenced hereinas “strong” control gates because the level of field coupling with thefloating gates on either side of them will usually, but not necessarily,be higher than the field coupling of the control gates 119, 121, 123etc. with their adjacent floating gates. The control gates 119, 121, 123etc. are therefore termed “weak” control gates. Factors responsible forthis difference in field coupling may include differences in couplingareas between the control and floating gates, and differences in thethicknesses and electrical characteristics of the dielectric between thecontrol and floating gates.

As can be seen from FIG. 3, a current path through a string of memorycells exists (a) along the surface 87 under the control gates 119, 121and 123, (b) along the side walls and portions of the bottoms of thetrenches 81 and 83 opposite the floating gates, and (c) along thebottoms of the trenches between the floating gates. The conductivity ofthis current path in region (b) is controlled by the voltage on thefloating gates. This floating gate voltage depends upon the amount ofelectron charge stored on the individual floating gate plus a voltagecoupled to the floating gate from voltages on the pair of control gatesthat is field coupled with each floating gate. The conductivity ofregions (a) and (c) can be controlled by one of two techniques. One isto provide doped regions 131-133 by implanting ions into the surface 87(for region (a)), preferably at an early stage before forming thetrenches 81 and 83. Doped regions 135 and 136 are also formed byimplanting ions in the bottom of the trenches 81 and 83 (for region(c)), at appropriate stages in the process. In this case, the dielectriclayers 115, 117 and 125 under the control gates are made relativelythick in order to minimize the coupling between the control gates andregions (a) and (c). An alternative technique omits the doped regions131-133 and 135-136 and makes the respective dielectric layers 115, 117and 125 relatively thin so that the control gates are sufficientlycoupled with regions (a) and (c) to control conduction through them bythe levels of voltages on the control gates. Further, it is possible touse one of these techniques in one of the regions (a) or (c) and theother technique in the other of these regions.

The floating gates are preferably formed as spacers. One process is tofirst cover the surface 87 with a thick layer of dielectric, such assilicon nitride, and then form an etch mask on top of this layer withthe pattern of continuous trenches extending in the x-direction.Trenches are then etched through this layer and into the surface 87.After formation of the dielectric layers 91, 93, 95 and 97 within theseelongated trenches, a layer of doped polysilicon is isotropicallydeposited over the array area and into the trenches. This layer is thenanisotropically etched back to leave continuous polysilicon spacersalong sidewalls of the trenches. These continuous spacers are thenseparated by an etching process that results in forming individualfloating gates with spaces between them in the x-direction. These spacesare between the NAND strings of FIG. 1. These spaces may later be filledwith a dielectric or, alternatively, a conductive material to shieldadjacent floating gates in the x-direction against electromagnetic fieldcoupling with each other.

A first set of control gates may then be formed in the trenches, such asthe control gates 101 and 103 of FIG. 3. After the floating gates arecovered by a dielectric layer, such as the layers 105, 107, 109 and 111,another layer of doped polysilicon is deposited over the array area andinto the trenches. Excess polysilicon outside of the trenches is thenremoved by etching or chemical-mechanical-planarization (CMP) to leavethe control gates in the trenches. Remaining portions of the thickdielectric layer that was initially deposited before the trenches wereetched is thereafter removed. Spaces are then formed between thefloating gates extending out of the trenches, in the view of FIG. 3.After any necessary dielectric layers are formed on any exposed surfacesof the floating gates and on the surface 87, a second set of controlgates may then be formed between the trenches, such as the control gates119, 121 and 123 of FIG. 3. These may be formed by depositing yetanother layer of doped polysilicon over the array area and into thesespaces, with polysilicon outside of these regions being removed byetching or CMP. Top thin portions of the floating gates may also beremoved by CMP in order to result in floating gates with more uniformthicknesses. In this case, the floating gate spacers are made higherthan they are expected to be after completion of the process and thethinner top portions thereafter removed.

Further steps in the process include formation of the control gate lines71-77. A conductive material, such as doped polysilicon, is depositedover the array area and then etched through a mask to leave theelongated strips 71-77 shown in FIGS. 2 and 3. This step may not beself-aligned with remaining portions of the structure but the structurethat exists at this point in the process allows a satisfactory alignmentof the mask with it.

Although doped polysilicon has been described for the floating andcontrol gates, and for the lines connecting them, a metal mayalternatively be used for some or all of them. Polysilicon is usuallypreferred because it can withstand the very high temperatures to whichthe structure is subsequently exposed during other processing steps.When a metal is used for an element, steps that take place after themetal is formed are modified so that they may be carried out at a lowertemperature than the melting point of the particular metal(s) used.

As a further alternative, rather than forming floating gates as thememory cell charge storage elements, a dielectric charge trappingmaterial may be substituted. The middle nitride layer of ONO, forexample, can also be deposited by the same spacer technique as describedabove for the floating gates. Uniform thickness layers of oxide areformed in a manner to be on either side of the nitride spacer to providethe ONO structure. Alternatively, spacers of silicon rich silicondioxide can be formed in the same manner as the conductive floatinggates, and the dielectric layers provided on either side of the floatinggates need not be formed on such dielectric spacers. As a furtheralternative, a five layer structure of tantalum nitride, aluminum oxide,silicon nitride, silicon oxide and silicon (TANOS) may be used to trapcharge. Since charge does not migrate across the dielectric storageelements, they are preferably programmed by a technique that distributescharge over an area similar to that of the floating gates shown.Further, a dielectric charge storage medium can be added across aconductive floating gate that is shown in the drawings, thereby forminga composite charge storage element where charge is stored in both thedielectric and conductive materials.

An equivalent circuit for the NAND string 23 of memory cells is shown inFIG. 4. The reference numbers used in FIGS. 2 and 3 also appear in FIG.4 to identify the same elements. Each floating gate is field coupledwith one of each of the two sets of control gates, one set within thetrenches and the other between the trenches, except perhaps for thefloating gates at the ends of the strings. As can be seen from FIG. 3,the level of coupling between a floating gate and its two control gatesmay not be the same. Taking the floating gate 30 as an example, it mayhave a different amount of area coupled with the control gate 103 on oneside than with the control gate 121 on the other side. The dielectriclayers between the floating gate 30 and each of the control gates 103and 121 can also have different thicknesses, be of different materialshaving different dielectric constants, or have other differences.

In one embodiment, programming the floating gates of the example memorycell system of FIGS. 1-3 may be accomplished by Fowler-Nordheimtunneling of electrons across the dielectric layers 91, 93, 95, 97 etc.,which separate the channels from their associated floating gates.Electrons are injected from the vertically oriented channel surface ofthe semiconductor structure to the associated floating gate. Appropriatevoltages are applied to at least the two control gates on opposite sidesof the floating gate being programmed to cause electrons to tunnelacross the channel oxide by sufficiently raising the floating gatepotential to create a strong enough electric field in the channel oxidethat will allow tunneling to occur. The selected cells for programmingon one row of the memory array will have both their strong and weakcontrol gates at very high voltage (e.g. 20V). The neighbor rows to theselected row share either the selected row's strong control gate or theselected row's weak control gate. The non-shared weak control gate orstrong control gate of these neighbor rows should be held at ground orat a negative voltage, if such voltage level is available, in order toprevent programming on these neighbor rows. It is preferable that eraseis performed through the same dielectric as programming was performed byreversing the electric field in the same dielectric. This can beachieved by holding the control gates at ground and raising the P-wellto a high voltage.

Alternatively, the floating gates may be programmed by Fowler-Nordheimtunneling of electrons through the gate dielectric residing between thefloating gate and the weak control gate. In this embodiment, theselected row of floating gates will have their strong control gate go toa high voltage (e.g. 20V), while their weak control gate is held atground or, if negative voltages are available, the weak control gate maybe taken to a negative voltage. Bit-line by bit-line selectiveprogramming will be achieved by boosting the NAND string channels of thecells that are to be programmed, and grounding the NAND string channelsof the cells that are to be inhibited. This is in contrast to what isdone when the programming is achieved by tunneling between the floatinggate and the channel where boosted channels resulted in program inhibitand grounded channels resulted in programming. In the presentembodiment, programming still includes raising the cell thresholdvoltages by inserting electrons onto the floating gates. Theavailability of three coupling terminals to each floating gate (namelythe strong control gate, the weak control gate, and the channel) allowsthe continued convention of cell-by-cell programming to consist ofadding electrons onto the floating gates. This is not performed in thetop tunneling NAND cell which has only two coupling electrodes for eachfloating gate.

The top tunneling NAND cell is described in detail in U.S. patentapplication publication No. 2006/0245245, entitled “Non-volatile MemoryCell using High-K Material and Inter-gate Programming.” In the memorydiscussed in this published application, programming on a bit-line bybit-line basis is carried out by defining programming as the act ofreducing cell threshold voltages by removing electrons from the floatinggates. Such a reversal of the typical definitions of programming anderasing is not necessary in the present embodiment.

Reading the level of charge on a particular floating gate of a NANDmemory cell string involves rendering the paths on either side ofsubject floating gate conductive by applying appropriate voltages to thecontrol gates. The subject floating gate in this embodiment is coupledwith control gates on opposite sides that in turn are also coupled withother floating gates. Techniques for reading the state of such anaddressed charge storage element are described in above-mentioned U.S.Pat. Nos. 6,888,755 and 7,075,823. A difference here, that may also needto be taken into account, is that the amount of coupling of the subjectfloating gate may be different with each of the two control gates on itsopposite sides along the string. If this unequal coupling exists,different voltages will likely be applied to the control gates than iftheir coupling with the subject floating gate was equal.

Specific NAND Array Embodiment

FIGS. 5-15 illustrate an example process in some detail for forming thearray of FIG. 2. With reference initially to the cross-sectional viewsof FIGS. 5A, 5B and 5C, the starting point is a silicon-on-insulator(SOI) wafer 150 that may be obtained from a wafer supplier. On a thickp-doped silicon base layer 151 is a silicon dioxide layer 153. On top ofthe insulating layer 153 is a silicon layer with a bottom thickness 155that is p+ doped, a middle region 157 that is p-doped and a top layer159 that is n+ doped, in this specific example. It is preferable toobtain the wafer from the supplier with the layer 155 already heavilyp-doped, since it is difficult to implant ions to this depth from thetop of the silicon layer. The n+ doping in the top layer 159 of the SOIlayer may be done either by the wafer supplier or by the user who isforming a memory cell array on the wafer. Layer thicknesses are shown inFIGS. 5A and 6A in nanometers (nm) for a very specific example but otherthicknesses can also be used.

FIG. 5A is a cross-section through one of the NAND strings of memorycells of FIG. 2, at section A-A thereof, FIG. 5B along one word line, atsection B-B of FIG. 2, and FIG. 5C is a cross-section along another wordline, at section C-C thereof. At the stage of the process illustrated inFIGS. 5A-B, all three views of the structure are the same. A layer 161of silicon dioxide has been formed on the top surface of the SOI wafer,preferably grown by thermal or radical oxidation but alternativelydeposited. A layer 163 of conductive material is then deposited over theoxide layer 161. The material of the layer 163 is typically dopedpolysilicon, primarily because polysilicon can withstand very hightemperatures to which the structure may be exposed during subsequentprocessing steps, but may also be a metal if the subsequent processingsteps are chosen to avoid such high temperatures. On top of theconductive layer 163, a hard mask material is formed of a silicon oxideor silicon nitride material, in a layer 165.

A next series of steps, the results of which are shown in FIGS. 6A, 6Band 6C, include patterning the mask material layer 165 by a standardphotolithography process to leave strips 165 a and 165 b elongated inthe x-direction (word line direction) across the array of FIG. 2, whilebeing spaced apart in the y-direction (bit line direction). There are,of course, a very large number of such strips, only two of which areshown for convenience. The layers under this mask are then etchedthrough the spaces between these strips, as shown. The etching continuesthrough the layer 159 of the SOI wafer and is stopped in about themiddle of the layer 157, as best shown by FIG. 6A. The distance etchedinto the layer 157 is sufficient to result in trenches in the wafer withdepths that provide the desired vertical channel length for the memorycells being constructed. But enough of the layer 157 is left so that n+ion implanted regions subsequently formed at the bottom of the trenchesare far enough above the p+ layer 155 so that formation of p-n junctionsare avoided since they would have the potential to break down duringoperation of the memory.

FIGS. 7A, 7B and 7C show the results of several additional processingsteps. A gate oxide layer 171 is deposited over the exposed horizontaland vertical surfaces. A second conductive layer, most commonly dopedpolysilicon in this example but could alternatively be a metal, is thenconformally deposited over the gate oxide layer to a thickness that isuniform on the horizontal and vertical surfaces (not shown). Thisconductive layer is then anisotropically etched to remove all of it fromhorizontal surfaces, thereby to leave vertically oriented strips 173 a,173 b, 173 c and 173 d that extend continuously across the array in thex-direction (word-line direction). These strips are later separated intoindividual memory cell floating gates.

A next step, the result of which is shown in FIGS. 8A, 8B and 8C, is toimplant n+ ions in regions 175, 176 and 177 along the bottoms of thetrenches that are continuously elongated in the x-direction across thearray. The implantation of arsenic is preferred since arsenic ionsdiffuse less during subsequent heating of the structure than dophosphorous ions. These n+ regions, as well as the n+ regions 159 a and159 b, provide conductive paths in the semiconductor structure alongNAND strings between vertical channels opposite floating gates alongsidewalls of the trenches.

A next series of processing steps separates the elongated walls betweenthe trenches into pillars. The result is shown in FIGS. 9A, 9B and 9C. Asuitable mask (not shown) is formed across the structure of FIGS. 8A-C.This mask has strips elongated in the y-direction across the array andspaced apart in the x-direction (see the plan view of FIG. 2). Materialof the structure between these mask strips is then etched away, throughall layers down into the oxide layer 153 of the SOI wafer, as shown inFIGS. 9B and 9C. This isolates the NAND strings from conduction betweenthem in the x-direction across the array. This characteristic of the SOIwafer provides very effective isolation and avoids the necessity ofadditional processing to do so. The result of this etching step is toform a two-dimensional array of pillars, such as pillar 181. The pillarsare surrounded on all four sides by trenches. The layers of the pillar181 are designated by the same reference numbers used for the layers onother views but with a “p” added. The first layer 165 of conductivematerial has now been separated into a first set of individual controlgates 165 p. The second layer 173 of conductive material has now beenseparated into individual floating gates 173 p 1 and 173 p 2 for thepillar 181.

Next, a dielectric is formed over all surfaces of the array, bothhorizontal and vertical, including bottoms of the trenches. A preferreddielectric is a three layer oxide-nitride-oxide (ONO) structure.Referring to FIGS. 10A, 10B and 10C, an oxide layer 185 is firstdeposited over the array, followed by a nitride layer 187 and thenanother oxide layer 189.

A next step, shown in FIGS. 11A, 11B and 11C, is to deposit a thirdconductive layer 191 of material over the tops of the pillars and whichfills in the trenches. This material may also be doped polysilicon,particularly if that has been used for the previous two conductivematerial depositions, but may alternatively be a metal, even ifpolysilicon has been used for the other conductive depositions, sincefewer processing steps remain at this stage that may need to beperformed at higher temperatures that a metal can withstand. Theadvantage of metal for the layer 191 is that it may melted immediatelyafter being deposited in order to assure that it conforms to thevertical walls of the trenches. The purpose of this third conductivelayer is to form a second set of control gates in trenches betweenfloating gates. Once this layer is deposited, the trenches surroundingthe pillars are then filled.

Excess amounts of the conductive layer 191 and dielectric layers underit are then removed over the entire array bychemical-mechanical-planarization (CMP). The result of the CMP step isshown in FIGS. 12A, 12B and 12C. Material is removed down to the hardmask layer 165, at which point the CMP process is stopped. Material fromthe third conductive layer now resides in all the trenches, surroundingthe pillars.

A next step is therefore to remove this conductive material between thepillars in the y-direction (in the direction of the bit lines). Theresult of this is shown in FIGS. 13A, 13B and 13C. Strips of a mask (notshown) are formed on the surface of the array in the state shown inFIGS. 12A, 12B and 12C to be elongated in the x-direction thereacrossand spaced apart in the y-direction. The masks extend across nowfilled-in trenches between pillars. Portions of the conductive layer 191between these mask strips are then etched away. This leaves the secondset of control gates 191 a, 191 b and 191 c in the trenches betweenfloating gates that are connected together across the array in thex-direction to form a set of control (word) lines extending continuouslyacross the array.

The first set of control gates 163 p of the pillars remain isolated. So,as shown in FIG. 14, they are connected by control (word) lines 201 and203 that are also elongated in the x-direction. A layer 205 ofdielectric is first formed over the array and then vias are formedthrough it and the mask material 165 p. A fourth layer of conductivematerial, preferably a metal, is then deposited over the layer 205 andinto the vias to contact the control gates 163 a, 163 p. The metal layeris then masked and etched to leave the control lines 201 and 203connecting one control gate of each NAND string across the array. Thedielectric layer 205 makes sure that the two sets of control lines arenot shorted or field coupled with each other.

It will be noted that one set 163 of control gates is marked on FIG. 14as “weak” and the other set 191 of control gates is marked as “strong.”This refers to the relative level of field coupling with the chargestorage elements on either side of the individual gates, as discussedabove. The control gates 163 have a smaller coupling area with theiradjacent floating gates than do the control gates 191, so this can causea lower level of coupling. Also, the thicknesses and compositions of thedielectric layers between the two sets of control gates and theiradjacent floating gates can be different. This coupling difference istaken into account when deciding on voltages to be applied to the twosets of control gates during programming and reading operations.

A few memory cells of the array resulting from the process of FIGS. 5-14are shown in plan view in FIG. 15. A significant advantage of thisstructure, and of the process of forming it, is its very compact nature.Many more floating gates can be fit into a given area of an integratedcircuit. The data storage density is therefore significantly increased.

CONCLUSION

Although the various aspects of the present invention have beendescribed with respect to exemplary embodiments thereof, it will beunderstood that the present invention is entitled to protection withinthe full scope of the appended claims.

1. A method of forming a non-volatile memory on a semiconductor wafer,comprising: forming a layer of conductive material over a surface of thewafer, forming trenches into a surface of the semiconductor waferthrough the conductive material layer, forming charge storage elementsalong walls of the trenches that extend out of the trenches and throughthe conductive material layer, forming conductive material within thetrenches that is field coupled with the charge storage elements formedalong walls thereof, separating the layer of conductive material and theconductive material within the trenches into individual control gates sothat charge storage elements are field coupled with control gates on twosides thereof, and forming control lines that connect the individualcontrol gates together.
 2. The method of claim 1, wherein thenon-volatile memory is formed on a wafer having a dielectric layer undera semiconductor layer, the trenches are formed into a portion of thesemiconductor layer and isolation is provided between certain of thememory cells by removing at least all the semiconductor layer betweenthe certain memory cells.
 3. The method of claim 1, wherein formingcharge storage elements comprises forming conductive floating gate.